Need to ask one question right up front: are you making backups to an external device? That is definitely
critical. If you are, what software are you using for that?
There are some things you can do on your own, along with getting some vey good free software for dong some cleanup. From a google search of "MacShiny", it seems like a bad product, along the lines of CleanMyMac and MacKeeper. Here is one of those "hits" from the search:
https://discussions.apple.com/thread/7402630?start=15&tstart=0
The first thing is to do as much disk cleanup on your own. This could involve getting rid of duplicates of files, stuff you no longer needed, etc. One possible source is to look inside your downloads folder.
Next, here are some excellent free applications you can download:
Onyx - available from here:
https://www.titanium-software.fr/en/onyx.html (make sure to get the correct version)
AppCleaner - available from here:
https://freemacsoft.net/appcleaner/
Malwarebytes - available from here:
https://www.malwarebytes.com/
GrandPerspective - available from here:
http://grandperspectiv.sourceforge.net/
Onyx is a good cleaning tool, has been around for a number of years, and has had excellent reviews. When you run it, execute the following tasks:
Under Maintenance: Permissions (you'll most likely need to run that a number of times)
Scripts
Rebuilding
Under Cleaning: Everything except Erasing
For AppCleaner, it does a very good job at removing both an application and all the associated "pieces".
Malwarebytes is good at finding whatever adware there is on your machine.
Finally, GrandPerspective provides a graphical way to seeing the space taken up by files on your machine.
Now, what third party applications do you use? And have you kept those up to date?
Next, when you have upgraded the Mac OS, did you do those upgrades "in place", ie, have the new Mac OS basically overwrite what was there already?
Finally, it might be wise to invest in a robust disk cleanup/maintenance/repair program, like Tech Tool Pro or Disk Warrior. I use both Onyx and Tech Tool Pro every week as part of my disk cleanup/maintenance/repair, and backup (use SuperDuper! for that) tasks, and I would not be without them.